Preheat oven to 400°F (200°C). Slice tomatoes into ½-inch rounds, arrange on a parchment-lined baking sheet, drizzle with olive oil, season with salt and pepper, and roast 25 minutes, turning once.

Fit the pie crust into a 9-inch dish, prick the bottom, line with parchment and weights, and blind-bake at 400°F for 15 minutes. Remove weights and bake 5 more minutes. Cool slightly and lower oven to 375°F (190°C).

Whisk eggs, cream, milk, Dijon, salt, pepper, and nutmeg. Stir in Gruyère and thyme.

Arrange roasted tomatoes in the crust, pour custard over them, and bake at 375°F for 35 to 40 minutes until the center jiggles gently. Tent crust with foil if it browns too fast.

Cool quiche on a wire rack for at least 15 minutes, then slice into wedges and serve warm or at room temperature.